The present invention relates generally to monitoring optical signals, more particularly, to a spectrometer and corresponding diffraction grating having improved performance.
The telecommunications industry has grown significantly in recent years due to developments in technology, including the Internet, e-mail, cellular telephones, and fax machines. These technologies have become affordable to the average consumer such that the volume of traffic on telecommunications networks has grown significantly. Furthermore, as the Internet has evolved, more sophisticated applications have increased data volume being communicated across telecommunications networks.
To accommodate the increased data volume, the telecommunications network infrastructure has been evolving to increase the bandwidth of the telecommunications network. Fiber optic networks that carry wavelength division multiplexed optical signals or channels provide for significantly increased data channels for the high volume of traffic. The wavelength division multiplexed optical channels or polychromatic optical signals comprise narrowband optical signals. The wavelength division multiplexed optical channels carry packets containing information, including voice and data. Contemporary optical networks can include forty or more narrowband optical channels on a single fiber and each narrowband optical channel can carry many thousands of simultaneous telephone conversations or data transmissions, for example. An optical component often utilized in performing a number of operations in optical networks is a diffraction grating.
Wavelength division multiplexed optical systems, such as systems utilizing diffraction gratings for performing multiplexing and demultiplexing operations, have the advantage of parallelism in transmitting optical signals. This yields higher performance and lower cost for high channel count systems. In particular, a diffraction grating is a device that diffracts light by an amount varying according to its wavelength. For example, if sunlight falls on a diffraction grating at the correct angle, the sunlight is broken up into its individual component colors (i.e., rainbow).
Gratings work in both transmission (where light passes through a material with a grating written on its surface) and in reflection (where light is reflected from a material with a grating written on its surface). In optical communications, reflective gratings have a widespread use. A reflective diffraction grating includes a very closely spaced set of parallel lines or grooves made in a mirror surface of a solid material. A grating can be formed in most materials wherein the optical properties thereof are varied in a regular way, having a period that is relatively close to the wavelength. Incident light rays are reflected from different lines or grooves in the grating. Interference effects prevent reflections that are not in-phase with each other from propagating.
There are two primary groove profiles in conventional diffraction gratings, blazed gratings and sinusoidal gratings. The blazed grating includes a jagged or sawtooth shaped profile. The sinusoidal grating has a sinusoidal profile along the surface of the grating.
The diffraction equation for a grating is generally described by
Gmxcex=n(sin (xcex1)+sin (xcex2))
where, G=1/d is the groove frequency in grooves per millimeter and d is the distance between adjacent grooves, m is the diffraction order, xcex is the wavelength of light in millimeters, xcex1 is the incident angle with respect to the grating normal, xcex2 is the exiting angle with respect to the grating normal, and n is the refractive index of the medium above the grooves.
FIG. 14A is a representative pictorial showing optical characteristics of a blazed diffraction grating in reflecting a narrowband optical signal. The blaze diffraction grating 900 is defined by certain physical parameters that effect optical performance. These physical parameters include the reflection surface material, the number of grooves g per millimeter, blaze angle xcex8B, and the index of refraction of an immersed grating medium 902. The reflection surface 905 typically resides on a substrate 910.
As shown on FIG. 14A, the groove spacing is defined by d. An incident narrowband optical signal with a center wavelength xcex1 has an incident angle xcex11 (measured from the grating normal Ng) and a reflection angle xcex21 (also measured from the grating normal Ng). The angle between the grating normal Ng and the facet normal Nf defines the blaze angle xcex8B.
As previously discussed, when light is incident on a grating surface, it is diffracted in discrete directions. The light diffracted from each groove of the grating combines to form a diffracted wavefront. There exists a unique set of discrete or distinct angles based upon a given spacing between grooves that the diffracted light from each facet is in phase with the diffracted light from any other facet. At these discrete angles, the in-phase diffracted light combine constructively to form the reflected narrowband light signal.
In practice, narrowband light signals or beams are not truly monochromatic, but rather a tight range of wavelengths. Each signal is defined by a narrow passband and has a center wavelength which is the representative wavelength to which an optical signal is associated. Each center wavelength is generally predefined, and may correspond with an industry standard, such as the standards set by the International Telecommunication Union.
A sinusoidal diffraction grating is similarly described by the equation above. When xcex1=xcex2, the reflected light is diffracted directly back toward the direction from which the incident light was received. This is known as the Littrow condition. At the Littrow condition, the diffraction equation becomes
xe2x80x83m*xcex=2*d*n*sin (xcex1),
where n is the index of refraction of the immersed grating medium 902 in which the diffraction grating is immersed.
FIG. 14B is a representative pictorial showing optical characteristics of a sinusoidal diffraction grating. Sinusoidal gratings, however, do not have a blaze angle parameter, but rather have groove depth (d). An immersed grating medium 955 resides on the sinusoidal grating surface 950 having a certain index of refraction, n. The diffraction grating equation discussed above describes the optical characteristics of the sinusoidal diffraction grating based upon the physical characteristics thereof.
FIG. 14c shows a polychromatic light ray being diffracted from a blazed grating 960. An incident ray (at an incident angle xcex8i to the normal) is projected onto the blazed grating 960. A number of reflected and refracted rays are produced corresponding to different diffraction orders (values of m=0, 1, 2, 3 . . . ). The reflected rays corresponding to the diffraction order having the highest efficiency (i.e., lowest loss) are utilized in optical systems.
An important component of the fiber optic networks is an optical performance monitor (OPM) for monitoring the performance of the optical system. The OPM provides a network/system operator the ability to monitor the performance of individual narrowband optical signals. The optical performance of the individual narrowband optical signals may include the following metrics, for example, power levels, center wavelength, optical signal-to-noise ratio (OSNR), interference between channels such as crosstalk, and laser drift. By monitoring these metrics, the optical network operator can easily identify and correct problems in the optical network so as to improve the performance of optical communication therein.
One form of an OPM employs a diode array spectrometer that generally includes optical lenses, a dispersion component, and an optical sensor. The optical lenses process the polychromatic optical signal and cause the polychromatic optical signal to be incident to the dispersion component preferably at a near-Littrow condition, which is an condition where the angle of the incident light beam is reflected back toward the source of the incident light beam near the incident angle at at least one wavelength. The dispersion component, typically a diffraction grating, diffracts the polychromatic optical signal into its narrowband optical signals, with each narrowband signal being diffracted at a distinct angle that is a function of the wavelength of the narrowband optical signal. Each narrowband optical signal forms a spot that is focused onto the optical sensor at a distinct location.
Ultimately, signal performance within an OPM device is attributable to a great extent to the performance of the diffraction grating therein. Because the parameter values which describe the diffraction grating often dictate the efficiency and the polarization effects of diffracted optical signals, much time, money, and effort have been dedicated to determining diffraction grating parameter values to effectuate improved optical performance. Due in part to the number of diffraction grating parameters, the considerable range of corresponding parameter values, and the interdependencies between the diffraction grating parameters, designing and implementing a diffraction grating yielding improved performance are nontrivial.
In this regard, designing diffraction gratings must additionally take into account real-world effects that can only be measured empirically to determine if the theoretical parameters for a diffraction grating yield a viable solution. For example, one difficulty in creating improved diffraction gratings is the prolonged time period for creating a master diffraction grating. A single diffraction grating master may take several weeks to produce. Although the master diffraction grating, having a specific set of grating parameters, may yield acceptable results (i.e., low loss or a partially polarization insensitive result), a replicated diffraction grating created from the master diffraction grating may produce less than desirable signal performance characteristics. Consequently, the process of designing and developing diffraction gratings (determining grating parameters that yield good signal and/or master grating related characteristics, producing a master diffraction grating having the determined grating parameters and producing a replicated diffraction grating from the master diffraction grating that yields good signal performance characteristics) so as to produce a diffraction grating having improved performance requires solving both theoretical and practical problems.
Based upon the foregoing, there is a need for a diffraction grating-based OPM having an improved optical performance for employment within an optical system.
Embodiments of the present invention are directed to an optical device for monitoring operating conditions of a multiplexed optical signal in an optical communications network. The optical device includes a diffraction grating in optical communication with input ports of the optical device so as to diffract multiplexed optical signals received at the input port as a demultiplexed optical signal having a plurality of narrowband optical signals over a wavelength range of at least approximately 30 nm. Within the wavelength range the optical device is substantially polarization insensitive. The diffraction grating may be a blazed diffraction grating or a sinusoidal diffraction grating.
The optical device may further include an optical detector optically coupled to the diffraction grating for receiving the narrowband optical signals and converting each narrowband optical signal into an electrical signal having a value representative of a power level of the corresponding narrowband optical signal.